When you separate the web app from the server and build it as a stand-alone React codebase, you can create applications that run in the browser, iOS, Android (via Cordova / reapp) and cross-platform desktop operating systems (via Electron).
Now wait a minute, you're going to say, I can do all of that with my server-side web application. And faster too. Sure, but does your application work when the user is offline? Does it have access to the client runtime's local storage for data persistence? Can it access client hardware like the camera, GPS, and accelerometer? Can it be distributed through Google Play and the Apple App Store? Can it be installed as a desktop application?
How do you use the same codebase across web, mobile, desktop when your app is going to access camera/gps/accelerometer which aren't available on desktop?
Now wait a minute, you're going to say, I can do all of that with my server-side web application. And faster too. Sure, but does your application work when the user is offline? Does it have access to the client runtime's local storage for data persistence? Can it access client hardware like the camera, GPS, and accelerometer? Can it be distributed through Google Play and the Apple App Store? Can it be installed as a desktop application?